1. Field of the Invention
This invention relates to stirring devices, and more particularly to a magnetic stirring device that is energized by a driving head of a microwave turntable.
2. Description of the Related Art
A chemical or biological sample in a container may be stirred by placing a stirring magnet or stirring magnetic device in a container with a sample and subsequently applying an external rotating magnetic field to create a corresponding rotation of the stirring magnetic device. The stirring magnet is typically a small bar magnetic material which can be encased in an inert plastic with a magnetic pole axis oriented in a horizontal direction. In some stirring magnets, an extension may be formed around a middle portion of the magnetic bar to thereby raise the stirring magnet above the bottom of the container and facilitate rotation of the magnet in the container. This is because the plastic extension exhibits a smaller surface area than the flat magnet.
In conventional magnetic stirrers, typically a motor mechanically rotates a drive bar magnet about a vertical axis of rotation. The drive bar magnet in turn rotates the stirring magnet in the container to thereby stir a chemical or biological sample. Where integrity of the sample must be maintained, a sterile stirring magnet may be sealed with the sample in the container so as to isolate the sample and stirring magnet from contaminating environments or particles. When the sample in the container requires heating, the continuous stirring of the magnetic stirrer helps to uniformly distribute the heat throughout the sample.
Nevertheless, heating of chemical or biological samples in microwave ovens has often been unpopular due to uneven spatial distribution of microwave energy in the oven cavity. As a result, "hot spots" and "cold spots" may be produced at different locations within the oven cavity, which may lead to unsatisfactory heating of the samples. Even microwave ovens with turntables can be ineffective in evenly distributing the heat within the sample. The turntables in many microwave ovens typically rotate at about five to six revolutions per minute, which is too slow for uniform distribution of heat in many chemical and biological samples. Ideally, the sample should be constantly stirred at a higher rotational speed during the heating process. This is necessary to uniformly distribute the heat throughout the sample while assuring that the sample is thoroughly mixed.
U.S. Pat. No. 5,593,609 to Fletcher discloses a mixing device for a microwave oven that has a gear assembly driven by a motor for moving a support plate in an orbital action during operation of the microwave oven. In one embodiment of this patent, the motor is a part of the microwave oven. In another embodiment of this patent, the mixing device can be self-contained with its own shielded motor and the assembly can be placed on the turntable of the microwave oven. During the heating mode, a container with a sample to be heated is placed on top of the support plate and both are moved in an orbital action during heating. In this patent, there is no provision for directly stirring the contents of the container by magnetic stirring bars for example. Moreover, this mixing device is not readily adaptable to be energized by turntable drive heads associated with microwave ovens.